I have seen a similar circuit used for a sleeping blanket controller. I bel=
ieve the triac is shown 'upside down' which won't help your understanding o=
f the circuit.

The sleeping blanket controller used an open collector comparator chip as t=
he controller (IIRC an LM324 or something similar) with a direct off the ma=
ins supply, resulting in a schematic similar to what you have shown. Becaus=
e the comparator is open collector it made sense to drive the triac between=
 the positive supply and the gate as otherwise the chip could not source en=
ough current to drive the triac gate. In your schematic this would require =
triac terminal 1 to be swapped with terminal 2, as the most sensitive gate =
drive arrangement is between terminal 1 and 3. I suspect your schematic sho=
uld also be drawn like this as well as otherwise the micro will have too hi=
gh a voltage on its output to survive.

I suspect the schematic you have supplied has been derived by someone who s=
aw or had used a similar schematic to what I described, and continued with =
the same system.

> Hello,
>=20
> I am a student, I saw this low voltage TRIAC driving circuit on the inter=
net and
> am puzzled as to why it uses a negative voltage rail?
>=20
> As far as I know, TRIACS are not sensitive to the polarity of the signal =
applied
> to the gate (with respect to MT1). Could this configuration work with a
> positive supply (with respect to neutral) in all four quadrants?
>=20
> --
> Jason White

--=20
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